In general, in an external combustion engine, such as disclosed in U.S. Pat. No. 4,077,221 (to Maeda), combustion takes place outside of a gas expansion chamber. A working gas is heated by the combustion, and enters the expansion chamber where the gas expands due to the heat and pushes a piston downward to deliver a power stroke. Through a linkage between the piston and crankshaft, the power stroke causes the crankshaft to rotate. After the power stroke, the piston moves upward and the gas is pushed out of the expansion chamber through an exhaust. In a closed cycle process, the exhausted gas is cooled and recirculated for use in the compression chamber, and later is reheated and re-enters the expansion chamber. In an open cycle, the gas is simply exhausted.
To make such an engine as efficient as possible, it is necessary to transform as much of the combustion energy as possible into heat energy of the gas. The longer the piston remains in the up position, the longer heat transfer between the combustion chamber and the gas can occur. It is thus desirable to create an external combustion piston-type engine in which the piston remains in the up position for a substantial portion of each cycle. It is also desirable to have the piston in the up position for a substantial amount of the cycle so that combustion can take place more thoroughly and therefore burn the fuel more efficiently.